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Cutworms Severe in SW ND (06/01/17)

Severe feeding injury and significant stand loss (several acres per field) from cutworms have been observed in canola, spring wheat, field peas and forages in southwest North Dakota this spring.

Cutworms Severe in SW ND

Severe feeding injury and significant stand loss (several acres per field) from cutworms have been observed in canola, spring wheat, field peas and forages in southwest North Dakota this spring. The photographs emailed to me show that the army cutworm is the main culprit! Large areas of cutworm damage have been reported in some fields. When food sources are depleted, army cutworms often move in large masses to new areas, thus the common name “army cutworms.”

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The army cutworm larvae are gray to brown with the back side (top) darker than the ventral side (bottom) with a pale mid-dorsal line. It is a climbing cutworm that “grazes” on the leaves of its host plants during the darkness of night. When foliar defoliation is low, plants can recover from army cutworm feeding if the growing point is not injured. Agronomists from SW ND indicated that field crops damaged from cutworms are sending up new tillers or side branches off the main stem. Most of the army cutworm larvae observed were about 1 inch long and getting close to maturity (1.5 to 2 inches long). If the majority of cutworms are mature, no control is recommended since larvae are at the end of their development and feeding period, and insecticide treatment would not be cost-effective. These larvae will be pupating (resting and non-feeding stage) in the soil soon. Army cutworm moths will emerge in June and fly to the Rocky Mountains for a period of inactivity and are a main food source of grizzly bears. From late August to late October, the moths become active and fly back to the plains to lay eggs in soft soil of freshly cultivated weedy fields or newly seeded winter wheat fields.

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Field scouting for cutworms is critical and should begin as soon as crops emerge, and fields should be checked at least twice per week until approximately late June or until the crop is no longer susceptible (past early growth stages). Cutworm damage symptoms are foliar defoliation or cut / wilted plants, leading to bare patches in the field. Examining 100 plants per five sampling sites by walking a ‘W or V’ pattern for a total of 500 plants in a field. Use a trowel to dig around damaged plants to determine if cutworms are present in soil or field debris. Missing plants in a row does not necessarily indicate cutworm larval damage; for example, gaps may be caused by a defective planter, poor germination, rodents or birds. The size of the cutworm larvae should also be estimated. Small larvae (<1 inch long) pose the greatest potential for crop damage as they still have to feed and grow larger.

The economic thresholds for cutworms vary by crop. Here’s some of the thresholds for North Dakota field crops:

Alfalfa – 3 to 4 larvae per square foot (new stands – only 2 per square foot)

Canola – 1 larvae per 3 feet of row

Chickpeas / Field Peas / Lentils – 2 to 3 larvae per square meter

Corn – 3 to 6 % of the plants are cut

Small grains – 4 to 5 larvae per foot of row; drought conditions 2 or more larvae per foot of row

Soybean - 1 larva per 3 feet of row or when 20% of plants are cut

Sugarbeet – 3 to 5 larvae per square foot or 4 to 5% cutting of seedlings (young beets)

Sunflower - 1 larva per square foot or 25 to 30% stand reduction

 

Several different insecticides are registered for cutworm control. Post emergent foliar insecticide treatment provides rapid knockdown of surface feeding cutworms. Most pyrethroid and organophosphate insecticides are contact insecticides. So, optimal control is achieved when insecticide applications are made at night when cutworms are active feeding. Wet soil conditions also will improve insecticide efficacy, as cutworms feed near the moisture line and soil surface in these conditions. Unfortunately, SW ND has not had any rain for over 2 weeks, so the impacts of cutworms will be more dramatic since they are feeding below the soil surface near the moisture line. This will reduce insecticide efficacy and increase crop stress from drought conditions and cutworm feeding injury.

Please consult the 2017 ND Field Crop Insect Management Guide. Insecticide seed treatment products, such as Cruiser (a.i. thiamethoxam), Gaucho (a.i. imidacloprid), and Poncho (a.i. clothianidin) are only labeled for cutworm suppression, not complete control. 

Janet J. Knodel

Extension Entomologist

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